Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobiles, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor.
For image sensors, it is a constant challenge to limit “hot pixels”, i.e. those pixels that output a signal even when there is no incident light. The hot pixel problem is particularly exacerbated during long integration periods (i.e. long exposure times). The hot pixel problem is closely related to dark current. As will be described further below, in the case of a four transistor (4T) pixel design, it is believed that the region beneath the transfer gate of the transfer transistor is a primary source of dark current.
One method to limit dark current is to apply a negative potential to the transfer gate. If the potential of the transfer gate is held negative, holes will accumulate under the transfer gate. This quenches electron generation which substantially reduces dark current and hot pixel effects. However, the application of a negative potential to the transfer gate results in reduced anti-blooming performance. At negative potential, the transfer gate is shut off completely. If there is a bright source of light incident on any of the pixels of the imaging array, then the generated photocharge on those pixels cannot spill over the transfer gate onto the floating node and from there over the reset transistor onto the Vdd drain. Instead, this high level of photogenerated charge spills onto adjacent photodiodes and results in a large white spot on the image (i.e. “blooming”).
If the potential on the transfer gate is held positive, on the other hand, there is an easy path through the transfer transistor and to the Vdd drain, since the transfer transistor is partially on. However, in this state, the well capacity of the photodiode is reduced.